The consequences of postpartum depression (PPD) extend far beyond the mother's mental health, and can have severe detrimental effects on marital relationships, parenting abilities, mother-infant bonding, and infant health and development. Although Latinos are the fastest growing population group in the US, few studies of PPD have been conducted with Latina women. Existing studies suggest a significantly elevated vulnerability to PPD for low income ethnic minority women. However, the trajectories of onset and recovery have not been well- described, and little is known about the influence of cultural-ecological risk and protective factors. Further, most research has failed to consider the fundamental role of the newborn baby and the ability of mother-infant interactions to influence the onset and course of PPD. This study will evaluate a community sample of 330 low-income Mexican American first- time mothers from the prenatal period through the first postpartum year. The development of PPD and the process of recovery will be examined using well-validated symptom and clinically diagnostic measures. Culturally-ecological factors will be examined that may either confer risk or offer protection from PPD. Repeated home observations of mother-infant interactions will be collected in the critical first 6 months following childbirth. The bio-psychosocial process by which mothers and infants co-regulate each other's emotions, behavior, and physiology will be analyzed and used to predict the longitudinal course of PPD over the first year. Findings will significantly enhance understanding of the impact of culturally-relevant risk and protective factors on the onset and course of PPD in a highly vulnerable population, and will identify innovative targets (e.g., mother-infant interactions) for future interventions. PUBLIC HEALTH RELEVANCE: Postpartum mood disorders, primarily depression, affect a large number of new mothers. Estimates range from 10-15 percent in general population samples. However, striking health disparities are evident in studies evaluating low-income and/or ethnic minority mothers, for whom prevalence rates from 24-49 percent have been reported. A large research literature documents the substantial detrimental public health impact of postpartum depression, not just for women, but for their partners and children as well. According to the World Health Organization (2007), depression is the leading cause of disability worldwide, and the fourth leading contributor to worldwide burden of disease. Infants and children of mothers who are depressed, especially those experiencing social disadvantage, face considerable short and long-term disadvantage, including lower birth weight, poorer cognitive development, higher rates of behavioral and social problems, and more frequent emotional problems. In Arizona, the largest percentages of new births are to Hispanic women, of whom the majority is classified as low-income. Population birth trends in Arizona (and the US as a whole), the magnitude of the public health impact of postpartum mood disorders, and the significant mental health disparities new Hispanic mothers experience argue for the critical need for further understanding of processes affecting the development of postpartum mood disorders in low-income Mexican American women.